The evolution of telecommunications networks has been such that the complexity and speed of networks in general have greatly increased. In addition to the development of network design, new and novel optical components are being brought to bear on the issues of speed and reach of optical channels. A particular aspect of network design is the need for dynamic configuration of networks. For example in an optical mesh many circuits based on an assignation of a specific wavelength are set up and reconfigured either for reasons of efficient traffic engineering or restoration. For a detailed description of optical network design see Journal of Lightwave Technology, Special Issue on Optical Networks, December 2000, vol. 18 pages 1606–2223 inclusive, the contents of which are incorporated by reference herein.
An enabling device for such next generation optical networks is a wavelength agile laser. A wavelength agile laser (hereafter “laser”) is a lasing device that may be tuned to either any discrete wavelength or arbitrarily tunable (or continuously tunable) within a given wavelength window. For telecommunications applications involving dense wavelength division multiplexing (hereafter DWDM) the wavelength range used is in what is known as the third window. The third window is the spectral region within which the attenuation exhibited by the transmission medium (commonly silica glass) is the lowest. Although loosely defined the third window may be identified to lie in the spectral region from 1500 nm to 1650 nm. Within this window the designations “S”, “C” and “L” represent subdivisions of this spectral region.
A requirement of tunable laser performance is therefore the capability to address the spectral region associated with S, C and L-band wavelengths. A further requirement of a tunable laser is that it is compliant with what is known as the “ITU grid”. The ITU grid is a defined standard covering the placement, in frequency space, of optical channels launched onto a fibre. In addition to the wavelength tunability requirements wavelength agile lasers must exhibit optical specifications compatible with high performance optical transmission. For a detailed description of the structure an optical performance requirements set on transmission lasers see J. Gowar, “Optical Communications Systems”, Second Edition, Prentice Hall International Series in Optoelectronics, pages 257 to 487, inclusive, the contents of which are incorporated herein as background.
An additional application enabled by tunable lasers is that of hardware restoration of an optical link in the event of the failure of a transmission source(s). A logical link may be assigned a specific wavelength from amongst a stream of optical wavelengths and, in order to protect every link, every wavelength must be protected individually. This leads therefore to the need for 100% redundancy in an optical transmission system and a consequent doubling of the equipment cost. The reason for this duplication in equipment is the very limited tunability of existing laser transmission sources. For a detailed description of the semiconductor based solutions to tunable laser solutions see V. Jayaraman et al., “Theory, Design and Performance of Extended Range Semiconductor Lasers with Sampled Gratings”, IEEE Journal of Quantum Electronics, vol. 29, no. 6, June 1993 the contents of which are incorporated herein as background. In addition see Session TuL from Optical Fibre Communications 2000, (OFC 2000) Technical Digest p. 177 onwards the contents of which are incorporated herein.
One approach to an electro-optically controllable filter is the work of Alferness et al., as described in U.S. Pat. Nos. 4,384,760, 4,390,236, 4,533,207, 4,667,331, 4,728,168, each of which is incorporated by reference herein in its entirety. A device of this type may also be seen in the article entitle “Narrow Linewidth, Electro-Optically Tuneable InGaAsP—Ti:LiNbO3 Extended Cavity Laser” by F. Heisman et al., Applied Physics Letters, 51, page 64 (1987), which is incorporated by reference herein in its entirety.
Wavelength selected polarization mode coupling is described in U.S. Pat. No. 5,499,256 issued to Bischel et al., which is also incorporated by reference herein in its entirety. Electrode structures disclosed in the above-described patents are similar to the ones disclosed by the Kaminow in U.S. Pat. No. 3,877,782 that is also incorporated by reference herein in its entirety.